Extensions to the charge exchange recombination spectroscopy diagnostic suite at ASDEX Upgrade

A new core charge exchange recombination spectroscopy diagnostic has been installed in the ASDEX Upgrade tokamak that is capable of measuring the impurity ion temperature, toroidal rotation, and density on both the low field side (LFS) and high field side (HFS) of the plasma. The new system features 48 lines-of-sight (LOS) with a radial resolution that varies from ±2 cm on the LFS down to ±0.75 cm on the HFS and has sufficient signal to run routinely at 10 ms and for special circumstances down to 2.5 ms integration time. The LFS-HFS ion temperature profiles provide an additional constraint on the magnetic equilibrium reconstruction, and the toroidal rotation frequency profiles are of sufficiently high quality that information on the poloidal velocity can be extracted from the LFS-HFS asymmetry. The diagnostic LOS are coupled to two flexible-wavelength spectrometers such that complete LFS-HFS profiles from two separate impurities can be imaged simultaneously, albeit with reduced radial coverage. More frequently, the systems measure the same impurity providing very detailed information on the chosen species. Care has been taken to calibrate the systems as accurately as possible and to include in the data analysis any effects that could lead to spurious temperatures or rotations.

A fast edge charge exchange recombination spectroscopy system at the ASDEX Upgrade tokamak

In this work, a new type of high through-put Czerny-Turner spectrometer has been developed which allows us to acquire multiple channels simultaneously with a repetition time on the order of 10 μs at different wavelengths. The spectrometer has been coupled to the edge charge exchange recombination system at ASDEX Upgrade which has been recently refurbished with new lines of sight. Construction features, calibration methods, and initial measurements obtained with the new setup will be presented.

Optimized tomography methods for plasma emissivity reconstruction at the ASDEX Upgrade tokamak

The soft X-ray (SXR) emission provides valuable insight into processes happening inside of high-temperature plasmas. A standard method for deriving the local emissivity profiles of the plasma from the line-of-sight integrals measured by pinhole cameras is the tomographic inversion. Such an inversion is challenging due to its ill-conditioned nature and because the reconstructed profiles depend not only on the quality of the measurements but also on the inversion algorithm used. This paper provides a detailed description of several tomography algorithms, which solve the inversion problem of Tikhonov regularization with linear computational complexity in the number of basis functions. The feasibility of combining these methods with the minimum Fisher information regularization is demonstrated, and various statistical methods for the optimal choice of the regularization parameter are investigated with emphasis on their reliability and robustness. Finally, the accuracy and the capability of the methods are demonstrated by reconstructions of experimental SXR profiles, featuring poloidal asymmetric impurity distributions as measured at the ASDEX Upgrade tokamak.

Imaging motional Stark effect measurements at ASDEX Upgrade

This paper presents an overview of results from the Imaging Motional Stark Effect (IMSE) diagnostic obtained during its first measurement campaign at ASDEX Upgrade since installation as a permanent diagnostic. A brief overview of the IMSE technique is given, followed by measurements of a standard H-mode discharge, which are compared to equilibrium reconstructions showing good agreement where expected. The development of special discharges for the calibration of pitch angle is reported and safety factor profile changes during sawteeth crashes are shown, which can be resolved to a few percent due to the high sensitivity at good time resolution of the new IMSE system.

Development of the gas puff charge exchange recombination spectroscopy (GP-CXRS) technique for ion measurements in the plasma edge

A novel charge-exchange recombination spectroscopy (CXRS) diagnostic method is presented, which uses a simple thermal gas puff for its donor neutral source, instead of the typical high-energy neutral beam. This diagnostic, named gas puff CXRS (GP-CXRS), is used to measure ion density, velocity, and temperature in the tokamak edge/pedestal region with excellent signal-background ratios, and has a number of advantages to conventional beam-based CXRS systems. Here we develop the physics basis for GP-CXRS, including the neutral transport, the charge-exchange process at low energies, and effects of energy-dependent rate coefficients on the measurements. The GP-CXRS hardware setup is described on two separate tokamaks, Alcator C-Mod and ASDEX Upgrade. Measured spectra and profiles are also presented. Profile comparisons of GP-CXRS and a beam based CXRS system show good agreement. Emphasis is given throughout to describing guiding principles for users interested in applying the GP-CXRS diagnostic technique.

High-resolution charge exchange measurements at ASDEX Upgrade

The charge exchange recombination spectroscopy (CXRS) diagnostics at ASDEX Upgrade (AUG) have been upgraded and extended to provide high-resolution measurements of impurity ion temperature, density, and rotation profiles. The existing core toroidal CXRS diagnostic has been refurbished to increase the level of signal, thus enabling shorter exposure times down to 3.5 ms. Additional lines of sight provide more detailed profiles and enable simultaneous measurements of multiple impurities. In addition, a new CXRS system has been installed, which allows for the measurement of poloidal impurity ion rotation in the plasma edge with high temporal (1.9 ms) and spatial resolution (down to 5 mm). A new wavelength correction method has been implemented to perform in situ wavelength calibrations on a shot-to-shot basis. Absolute measurements of the poloidal impurity ion rotation with uncertainties smaller than 1.5 km/s have been obtained. Comparison of all the CXRS measurements provides a consistency check of the diagnostics and good agreement has been found for all of the CXRS systems.

Intrinsic toroidal rotation, density peaking, and turbulence regimes in the core of tokamak plasmas

Observations in the ASDEX Upgrade tokamak show a correlation between the gradient of the intrinsic toroidal rotation profile and the logarithmic gradient of the electron density profile. The intrinsic toroidal rotation in the center of the plasma reverses from co- to countercurrent when the logarithmic density gradients are large, and the turbulence is either dominated by trapped electron modes or is at the transition between ion temperature gradient and trapped electron modes. A study based on local gyrokinetic calculations suggests that the dominant trend in the observations can be explained by the combination of residual stresses produced by E × B and profile shearing mechanisms.

First observation of edge localized modes mitigation with resonant and nonresonant magnetic perturbations in ASDEX Upgrade

First experiments with nonaxisymmetric magnetic perturbations, toroidal mode number n=2, produced by newly installed in-vessel saddle coils in the ASDEX Upgrade tokamak show significant reduction of plasma energy loss and peak divertor power load associated with type-I edge localized modes (ELMs) in high-confinement mode plasmas. ELM mitigation is observed above an edge density threshold and is obtained both with magnetic perturbations that are resonant and not resonant with the edge safety factor profile. Compared with unperturbed type-I ELMy reference plasmas, plasmas with mitigated ELMs show similar confinement, similar plasma density, and lower tungsten impurity concentration.

Evidence for strong inversed shear of toroidal rotation at the edge-transport barrier in the ASDEX upgrade

The toroidal rotation of H-mode plasmas in ASDEX Upgrade is studied in the outermost 5 cm of the confined plasma. The projection of the rotation velocity along the line of sight (approximately toroidal) is measured using charge exchange recombination spectroscopy, with a radial resolution of up to 3 mm and a temporal resolution of 1.9 ms. At about 1 cm inside the separatrix the rotation exhibits a local minimum. From there, the rotation in codirection increases towards the plasma center and towards the separatrix. The latter increase is the focus of this work. It is situated in the region of the edge transport barrier and amounts to 10-20 km/s. It is observed for D+, He2+, B5+, and C6+. The described rotation feature at the edge is not visible during an ELM crash and is probably connected to the occurrence of steep gradients in this plasma region.